[mips] Support 64-bit offsets for lb/sb/ld/sd/lld ... instructions

The `MipsAsmParser::loadImmediate` can load immediates of various sizes
into a register. Idea of this change is to use `loadImmediate` in the
`MipsAsmParser::expandMemInst` method to load offset into a register and
then call required load/store instruction.

The patch removes separate `expandLoadInst` and `expandStoreInst`
methods and does everything in the `expandMemInst` method to escape code
duplication.

Differential Revision: https://reviews.llvm.org/D47316

llvm-svn: 333774